1. Field of the Invention
This invention relates to a high heat resisting and high abrasion resisting aluminum alloy and aluminum alloy powder having superior toughness, abrasion resistance, high temperature strength, and creep resistance, which are useful to form engine parts such as valve spring retainers, intake valves, and pistons, etc. for automobiles, air planes, and the like.
2. The Prior Art
Owing to the light weight and superior processability, aluminum alloys have been used as structural members for airplanes and automobiles from old times. Particularly, Al-Cu-Mg alloys such as JIS 2024, 2018 according to Japanese Industrial Standards are known as heat resisting alloys.
An Al-Ni alloy including 5% by weight (hereinafter % means % by weight) of nickel has been proposed in pages 58 and 70 of the preliminary typescript pamphlet of the aluminum alloy powder metallurgy symposium held by the Light Metal Institute on Mar. 9, 1987. Japanese Unexamined Patent Publication (KOKAI) Nos. 149629/1990, 149631/1990, 149632/1990, and 149633/1990 disclose `Low Thermal Expansion Aluminum Alloys Having Superior Abrasion Resistance and Heat Conductivity` formed of Al-Ni-Si-Cu-Mg alloys including 8% or more of nickel and produced by casting.
Further, Japanese Examined Patent Publication (KOUKOKU) No. 56401/1990 discloses `High Heat Resisting, High Abrasion Resisting and High Tensile Aluminum Alloy Powder` formed of Al-Ni-Si alloy powder including 7.7 to 15% of nickel and 15 to 25 % of silicon and having a primary Si size of 15 microns or less.
Aluminum alloys often cause seizing as compared to steels, when in slidable contact with an aluminum alloy or steel. On purpose to improve the sliding characteristics, Japanese Unexamined Patent Publication (KOKAI) No. 88819/1979 discloses a cast aluminum alloy including 0.4 to 5.5% of boron, and Japanese Unexamined Patent Publication (KOKAI) No. 247334/1988 discloses a cast aluminum alloy including 0.5 to 10% of boron. Further, a refined cast aluminum alloy including titanium and about 0.05% of boron is also known.
An automotive engine is always required of high power and a light weight. Accordingly, materials of engine parts must have a tensile strength of 500 MPa or more at room temperature, and 160 MPa or more at 300.degree. C. It is preferable that the materials have a tensile strength of 200 MPa or more at 300.degree. C. It is also demanded that the materials cause no seizing and no fretting fatigue phenomenon when in slidable contact with metals such as steel component parts.
Seizure is a phenomenon showing a sliding characteristic of a sliding metal member, namely, a phenomenon that, when a sliding member repeatedly moves over a mating metal member under a high load, the friction generates excessive heat so that a portion of the sliding member is fused and adheres to the mating member, the friction coefficients of the both members drastically increase, and the sliding member is fixed to the other member. A Fretting fatigue phenomenon is also a phenomenon showing a sliding characteristic of a sliding metal member, namely a phenomenon that, when a sliding metal member repeatedly moves over a mating member under a high load even under a sufficient oil lubrication, a portion of the sliding member adheres to the mating member and a fatigue failure occurs from the adhered portion.
In the above views, the Al-Cu-Mg alloys such as above JIS2024, JIS2018 have a superior tensile strength at room temperature but a low tensile strength of 300 MPa at a high temperature of 200.degree. C., and a low tensile strength of 150 MPa at a high temperature of 300.degree. C. Therefore, these Al-Cu-Mg alloys cannot be used for engine parts of modern automobiles and the like. The above Al-Ni alloys and Al-Ni-Si-Cu-Mg alloys have improved heat resistance and abrasion resistance owing to the intermetallic compound NiAl.sub.3 generated in the tissue. However, because products are made by casting, the particle diameter of the intermetallic compound in the products become larger, and as a result, the tensile strength is only 380 MPa at room temperature and as low as 160 MPa at a high temperature of 300.degree. C. Therefore, even these alloys are difficult to be used for engine parts of modern automobiles and the like.
On the other hand, the above Al-Ni-Si alloy powder is made into products by sintering. Namely, a raw material alloy of a certain composition is melt and atomized, thereby producing the above Al-Ni-Si alloy powder, and then the Al-Ni-Si alloy powder is made into products by cold preforming, extruding, and forging. Therefore, in the Al-Ni-Si alloy powder the intermetallic compound NiAl.sub.3 has a particle diameter of about several microns or less, and the abrasion resistance is superior and the tensile strength is 510 MPa at room temperature and 345 MPa at 250 .degree. C. Sometimes, however, the Al-Ni-Si alloy cannot have a sufficient extruding ratio (a ratio of cross sections of the material before and after extruding) in producing engine parts of automobiles and the like.
Further, the cast aluminum alloy of Japanese Unexamined Patent Publication (KOKAI) No. 88819/1979 do not exhibit sufficient sliding characteristics, as it is assumed that boron does not exist as a simple substance. Namely, in a casting method, the amount of boron solid solved in the matrix of an aluminum alloy is small. Boron hardly dissolves at room temperature, and when gradually cooled, most of the boron dissolved in the molten metal varies into boron compounds such as AlB.sub.12. It is supposed that these facts prevent the cast aluminum alloy from exhibiting sufficient sliding characteristics.
Further, MMC (metal matrix composites) in which ceramic particles or fibers are dispersed generally have a high strength at elevated temperatures, but a low forgeability and a low elogation. When we aim to improve the forgeability of aluminum alloys, high temperature strength decreases instead. Therefore, an optimum matrix must be selected for an aluminum alloy in order to have both a high strength at elevated temperatures and a superior forgeability.